Method of preparing orthosilicic acid alkyl esters

ABSTRACT

A process for preparing an orthosilicic acid tetraalkyl ester having 2 to 6 carbon atoms in the ester group which comprises contacting metallic silicon with an alcohol corresponding to the ester group in the presence of the corresponding alkali alcoholate: 
     A. in the presence of a surface active substance; 
     B. in the presence of a compound containing a methoxy group; or 
     C. under pressure at a temperature above the boiling point of the reaction mixture.

This is a division of application Ser. No. 706,864, filed July 19, 1976,now U.S. Pat. No. 4,113,761, issued Sept. 12, 1978.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for preparing orthosilicic acidtetraalkyl esters having 2 to 6 carbon atoms in the ester group. Moreespecially, this invention relates to a process for preparing suchorthosilicic tetraalkyl esters which process is characterized byexceptionally improved volume-time yields. This invention isparticularly directed to a process for preparing such an orthosilicicacid tetraalkyl ester wherein the process is carried out in the presenceof a surface active substance or a compound containing a methoxy groupor is carried out under pressure at a temperature above the boilingpoint of the reaction mixture. The invention is concerned especiallywith methods for obtaining orthosilicic acid alkyl esters in highervolume-time yields than has been provided by former methods.

2. Discussion of the Prior Art

It is known to prepare orthosilicic acid tetraethyl esters in accordancewith West German Pat. No. 1,768,781 by reacting metallic silicon withethanol in the presence of high-percentage alkali ethylate solutions.This method has the disadvantage that relatively low volume-time yieldsare achieved by the application thereof.

The above-mentioned disadvantages are offset to a certain extent by themethod of West German Pat. No. 1,793,222. In this method the reaction isperformed in the presence of the desired orthosilicic acid tetralkylester. This brings about a lower concentration of the alkali alcoholatesin the reaction mixture.

Both methods are very well suited for the preparation of silicic acidtetramethyl ester. In the preparation of silicic acid tetraethyl esterand of the higher esters, however, difficulties are encountered whichresult in a reduction of the material yield and of the volume-timeyield. The causes of these difficulties remain unexplained to this date.

However, in the preparation of silicic acid tetraethyl ester and thehigher esters, with the silicon placed first in the reactor, the speedof the reaction diminishes constantly in the course of the reaction, andthe components do not react completely, for this reduction of speed ismore rapid than the reduction of the concentration of the components. Inthe preparation of silicic acid tetraethyl ester, for example, theresult is that only about 40% of the silicon charged enters thereaction, and then, despite an excess of ethanol, the reaction comes toa halt.

While not wishing to be bound by any theory, it is believed by some thatthe decrease of the speed of reaction during the process is explainablein that products form during the process which adversely affect thesurface of the metallic silicon. This adverse affect on the surface ofthe silicon, it is believed, is eliminated pursuant to the invention.

It is an object of this invention, therefore, to provide a process whichspeeds the course of the reaction of metallic silicon with a C₂ -C₆alcohol en route to the preparation of orthosilicic acid tetraalkylesters having 2 to 6 carbon atoms.

SUMMARY OF THE INVENTION

Broadly, this invention contemplates a process for preparing anorthosilicic acid tetraalkyl ester having 2 to 6 carbon atoms in theester group which comprises contacting metallic silicon with an alcoholcorresponding to the ester group in the presence of the correspondingalkali alcoholate:

A. in the presence of a surface active substance;

B. in the presence of a compound containing a methoxy group; or

C. under pressure at a temperature above the boiling point of thereaction mixture.

It has been observed, in accordance with this invention, that improvedvolume-time yields are effected if the preparation of the orthosilicicacid tetraalkyl ester is performed either (A) in the presence of a knownsurface active substance, and/or (B) in the presence of a compoundcontaining a methoxy group and/or (C) by performing the reaction underpressure at a temperature above the boiling point of the reactionmixture. It has been observed that no disadvantageous effect is impartedto the surface of the silicon reactant if the process is carried out insuch a manner. An increase in the speed of the reaction is effectedsurprisingly. For instance, when the process is carried out underpressure at a temperature above the boiling point of the reactionmixture, the increase in the speed of the reaction is greater thanaverage in comparison with known thermodynamic influences.

The compounds containing methoxy groups and the surface activesubstances can be added at any time during the reaction. If they areadded at a time when the speed of the reaction has already greatlydiminished an increase of the speed is soon obtained which isconsiderably greater than that which prevailed at the onset of the pureethyl ester reaction. This increase cannot be explained simply on thebasis of a transesterification resulting in a parallel formation ofmethyl ester from methanol and the metallic silicon, because the speedachieved in the reaction is greater than the sum of these individualreactions.

The addition of the surface active substances at the onset of thereaction causes the reaction to take place between silicon and alcoholwithout appreciable impediments, and the difficulties described aboveare not encountered or encountered only to a reduced extent.

Suitable surface active substances useful herein include known wettingagents, emulsifiers, penetrating agents or flotation agents, providedthat they do not enter into conflicting reactions with one of thereactants. The amount to be added depends on the nature of thesubstances used. In general, small amounts, amounting to 2.5 to 0.01% byweight Si and generally about 1% of the weight of the metallic siliconcharged suffices, as long as it is sufficient to cover the surface ofthe silicon metal.

The surface active substances useful in accordance with the inventionalso include nitrogen-containing organic bases, such as cyclic tertiaryor secondary amines, especially C₁ -C₈ tertiary or secondary cyclicamines (e.g., quinoline, isoquinoline, pyridine, piperidine) ordiarylsulfoxides, dialkylenesulfoxides, and dialkylthioureas. The use ofthese compounds has the additional advantage that they more or lessgreatly counteract the undesirable secondary reaction of the formationof alkanes and water, which will be described further below.

If flotation agents are used as surface active substances, it canhappen, especially if foaming agents are also used, that the reactionmixture will foam up, and most of the metallic silicon will gathertogether with the foam in the upper part of the reaction vessel. It isthen desirable to allow the foam to overflow together with the siliconthrough a suitable overflow device into a second reaction vessel, andthere to continue the reaction by the addition of a defoaming agent andfreshly added alcohol, alcoholate and, if desired, silicic acidtetraalkyl ester. The silicon that overflows no longer displays theabove described reluctance to enter the reaction, and reacts likefreshly introduced silicon.

Methanol, alkali methylates and silicic acid methyl ester are especiallysuitable as compounds containing methoxy groups. Of these, the alkalimethylates, such as sodium methylate, for example, have shown thestrongest action. The amount to be added can be between 5 and 20%,preferably between 8 and 15%, of the weight of the metallic silicon,reckoned as --OCH₃ groups. The alkali methylates can also be added insolid form.

In the reaction between silicon and alcohols, it is known that hydrogenforms in statu nascendi and to a slight extent hydrogenates the alcoholpresent, to form alkanes and water (cf. Houben-Weyl VI/2, p. 100). Thiswater reacts with the catalyst, diminishing its activity, and it reactsalso with the silicic acid ester that has formed in the alkalinereaction solution. If the reaction is performed under pressure, onewould have to expect that this hydrogen gas, which is not carried off inthe reaction is performed under pressure, would shift the reactionequilibrium more towards the starting components. Surprisingly, however,this is not the case.

The higher the pressure is, the more pronounced is the reaction betweenthe hydrogen formed in the reaction and the alcohols whereby water isformed. This intensified formation of water theoretically acts againstany increase in the speed of the reaction in the case of the reactionbetween silicon and alcohol under pressure and at high temperatures.Actually, however, the opposite is the case.

The reaction between silicon and alcohol in the presence of alkalialcoholates generally starts up at temperatures around 160° C. Up tothis temperature the system can be kept closed if it is operated underpressure. After the reaction starts up, the exothermia of the reactionmakes additional heat input unnecessary. After the onset of thereaction, the increase in the pressure is no longer proportional to theincrease of the reaction temperature but overproportional due to thehydrogen that is formed in the reaction. In principle, one can performthe reaction at pressures up to 50 atmospheres gauge. In general, thedesired pressure depends on the apparatus available. When the desiredpressure is achieved, it can easily be kept constant by letting off thenewly formed hydrogen through suitable valves. Letting off the hydrogenpermits control of the reaction temperature. Since a mixture of alcoholand silicic acid ester is also escaping, evaporation heat is also beingremoved from the system.

The above-described release of the alcohol-silicic acid mixture alsopermits the process to be performed continuously under pressure. In thiscase it is desirable to feed in the silicon as a dispersion in alcoholor silicic acid ester. The amount of the alcohol used in that case, andthe ratio of silicon to alcohol, is governed by the amount ofalcohol-silicic acid ester mixture that is withdrawn. Essential to acontinuous process is the establishment of a temperature at which thereaction mixture will boil at the selected pressure. The boiling pointof the mixture can be controlled through the alcohol content.

In principle, the reaction is performed in the manner described inGerman Pat. Nos. 17 68 781 and 17 93 222, the disclosures of which arehereby incorporated herein as references. The concentration of theindividual components in the system can vary within wide limits. It isadvantageous, however, to select the proportions such that an easilystirred mixture is obtained, and the ethylate is present as a solute inthe alcohol. Basically, however, one can operate without an excess ofsilicon in the system and feed in the silicon together with the alcoholonly in the amount that will react per unit of time.

The reaction can be performed discontinuously or continuously. Theseparation of the reaction product from the reaction mixture ispreferably performed by distillation.

The water that forms in small amounts due to the secondary reactiondescribed above reacts with the catalyst, which thus loses some of itsactivity, and also with the silicic acid ester formed in the alkalinereaction solution. The removal of the water can be accomplished bypartially distilling the reaction mixture, using as the withdrawingagent the hydrogen gas that is forming, or also an inert gas, such asnitrogen for example, which is additionally passed through the reactionvessel.

The water-containing, gaseous distillation products are at the same timecondensed in such a manner that the condensate will be unable to flowback directly into the reaction vessel. The return of the distillate tothe reaction vessel is then accomplished, if desired, by means ofappropriate water withdrawing agents.

If, in the case of a continuous procedure, the methoxy groups in theform of the methanol that is formed are removed from the reactionmixture during the distillative separation of the ester, they must bereplaced continually by the addition of fresh material. If the mixedesters that form due to the addition of the compounds containing methoxygroups are not desired, the distillative separation is performed bymeans of a column in which the methoxy groups are carried away in theform of methanol as a result of transesterification with excess ethanol.

The small amount of catalyst that is consumed during the reaction asdescribed above can be replaced together with the alcohol feed or withthe feed of the surface active substance.

Pure silicon as well as ferrosilicon or other silicon alloys containingmore than 50% silicon can serve as the metallic silicon to be used inaccordance with the invention. The grain size of the silicon or siliconalloys should be no greater than 100μ. Preferably it is between 2 and20μ.

In order to more fully illustrate the nature of the invention and themanner of practicing the same, the following examples are presented:

EXAMPLE 1 (for purposes of comparison)

In a one-liter stirring vessel having an anchor stirrer running alongthe walls and equipped with a temperature measuring means and a refluxcondenser from whose open end the hydrogen that forms during thereaction is removed and measured by means of a gas meter, 200 grams ofFeSi of a fineness between 15 and 20μ, 400 grams of silicic acidtetraethyl ester, and 20 grams of sodium ethylate are combined. Afterthe addition of 100 grams of ethyl alcohol, the mixture is heated to therefluxing temperature.

After the reflux temperature is reached, an hourly formation ofapproximately 6 liters of hydrogen is established. This corresponds to avolume-time yield of 28.5 grams of silicicic acid tetraethyl ester perliter per hour.

EXAMPLE 2

In a one-liter stirring autoclave equipped with a temperature measuringmeans and a pressure-tight reflux condenser at whose upper end there isprovided a regulating valve for controlling the pressure and the removalof the hydrogen, 200 grams of FeSi of a fineness between 15 and 20μ, 400grams of silicic acid tetraethyl ester, 20 grams of sodium ethylate and100 grams of ethyl alcohol are combined. The autoclave is preheated to150° C.; then the heating system is turned on. Then the temperatureincreases within 5 minutes to 185° C. while the pressure rises to about10 atmospheres absolute. This pressure is maintained constant by lettingoff the hydrogen that continues to form, through the balancing valve andthrough the gas meter. The reaction was sustained for another 20minutes, the temperature rising to 198° C. By the end of these 20minutes a total of 21 liters of hydrogen had been let off. The amount ofhydrogen formed corresponds to a volume-time yield of about 280 grams ofsilicic acid tetraethyl ester per liter per hour. This proves that themethod of the invention produces a considerable increase of thevolume-time yield in comparison with the known methods of Example 1.

EXAMPLE 3

In a one-liter stirring vessel equipped with an anchor stirrer runningalong the walls, an insulated distillation column, an input connection,and a system for measuring the temperature in the liquid, 500 grams ofsilicic acid tetraethyl ester, 250 g of FeSi and 12.5 g of sodiumethylate are combined. After the boiling temperature is reached, ethanolis fed in. The reaction that starts results in esterification at a rateindicated by the formation of hydrogen gas at 6 liters of hydrogen gasper hour. After one hour of operation, the rate of the reaction drops tofrom 2 to 3 liters H₂ /h.

Then 25 g of silicic acid tetramethyl ester (=8 wt-% methoxy groups withrespect to FeSi) is added to the hot reaction mixture. The formation ofhydrogen immediately increases to 24 l/h and remains at this level forseveral hours while the addition of ethanol continues. This correspondsto a 300% increase in the speed of the reaction.

After the addition of another 10 g of silicic acid methyl ester (totaladdition approximately 11% methoxy groups), the rate of the reaction canbe increased to 32 l H₂ /h for several hours while continuing theethanol feed.

When the ethanol feed is terminated, the excess alcohol reacts away, anda boiling point of 163° C. establishes itself. This boiling point isonly slightly below the theoretical boiling point of the tetraethylester of 165° C. The reduction of the boiling point is to be attributedto a small content of mixed esters.

After several hours of shut-down, the mixture was again heated to theboiling point and the ethanol feed was resumed, and the same rate ofreaction was achieved as before the ethanol feed was shut off.

EXAMPLE 4

The procedure was the same as in Example 3, except that sodium methylatewas added instead of silicic acid tetramethyl ester, in the amount of 40grams (=9 wt.-% methoxy groups). 180 liters of hydrogen gas per hourdeveloped momentarily; after the reaction had been sustained for severalhours at a uniform rate of ethanol feed, the reaction rate, as measuredby the formation of hydrogen, decreased to from 80 to 100 liters ofhydrogen gas per hour. Upon the addition of another 20 g of sodiummethylate, the hydrogen gas formation could be increased again to from180 to 200 l/h.

What is claimed is:
 1. A process for preparing an orthosilicic acidtetraaalkyl ester having 2 to 6 C atoms in the ester group whichcomprises contacting metallic silicon with an alcohol corresponding tothe ester group in the presence of the corresponding alkali alcoholateand orthosilicic acid tetraalkyl ester having 2 to 6 carbon atoms in theester group under pressure and in the liquid phase at a temperatureabove the boiling point of the reaction mixture.
 2. A process accordingto claim 1 wherein the process is performed at a temperature between160° and 210° C. under pressure up to 50 atmospheres gauge.
 3. A processaccording to claim 2 wherein the pressure is regulated by bleeding offhydrogen formed during the reaction.
 4. A process according to claim 2wherein the process is conducted continuously, silicon is introducedinto the reactor in the form of a dispersion in alcohol or in thedesired silicic acid ester and the formed ester is withdrawn togetherwith hydrogen formed in the process in a mixture with alcohol.
 5. Aprocess according to claim 1 wherein the reaction mixture initiallycontains orthosilicic acid tetraalkyl ester having 2 to 6 carbon atomsin the ester group before the metallic silicon is reacted with thealcohol.